On locking the screen with XScreenSaver I noticed that a popup window from a chat was able to show up and thus leaking private information. On reporting this against XScreenSaver it seems that the X-Server does not provide the ability to uniquely claim the top drawing area in a way that all other applications can't draw on top of it anymore to counter such issues.
But similar to bug #69298 comment #1 it could also become an issue if applications begin to uniquely claim the top drawing area in case this is unwanted by the user (for example a fullscreen game could do this and more or less unintentionally preventing the user to add his custom overlays). Eventually this needs also to be bind on higher permissions so that this can be configured.
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